Polymeric networks formed through the action of UV/visible radiation are generally constructed via a free-radical reaction initiated by small-molecule photosensitive activators. These networks can be based on either crosslinkable, preformed polymers or by reactions of monomers with di- and tri-functional crosslinking agents. Crosslinked polymeric networks are found in a wide variety of industrial and medical uses.
Recently, there has been significant interest in synthesizing crosslinked hydrophilic polymers for various biological uses. Hydrogels (that is, crosslinked hydrophilic polymers) generally, because of their characteristic properties such as swellability in water, hydrophilicity, biocompatibility and lack of toxicity, have been utilized in a wide range of biological and medical applications. The anti-thrombogenic and biocompatibility properties of poly(ethylene glycol) (PEG), for example, have been extensively studied and are well established. In the case of existing PEG-based networks, the crosslinkable groups are traditionally acrylate or methacrylate groups, which readily polymerize in the presence of 365 nm radiation and the appropriate activators. The carbon-carbon bonds formed during the cross-linking reaction are relatively permanent, depolymerizing only at elevated temperatures which would likely destroy the entire material. In a number of applications, however, it would be beneficial to develop reversibly cross-linked hydrogels.
Although conventional chemical crosslinking has been extensively used as a hydrogel preparation method, relatively little work has been reported on the preparation of hydrogels via photopolymerization of water soluble polymers. Current photoinduced systems for hydrogel preparation include: (i) free radical polymerization initiated by long wave ultraviolet light or visible light of acrylate groups attached to water soluble polymers, and (ii) photopolymerization of photosensitive groups such as cinnamate, stilbazolium or coumarin which are pendant to the end of hydrophilic polymers. Although such work has demonstrated photopolymerization, there has been no demonstration of a truly photoreversible system. Indeed, only very limited photoreversibility has been observed.
In that regard, at least one study has reported a photopolymerized system comprising a water soluble polymer having a cinnamate pendant group, but the extent of photoreversibility of that system, as observed by UV spectroscopy, was less than 25% (as measured by the change of the absorbance at 275 nm). In addition, the time of irradiation that was required to detect such reversible behavior was quite long (that is, on the order of 2 to 3 hours). Likewise, coumarin groups have been reported to produce photocrosslinked polymer systems based on polyoxazolines. A 55% conversion of the photoinduced dimer to the starting material has been reported for such polyoxazoline-based systems. The conversion was calculated from the UV absorbance spectrum after irradiation of the polyoxazoline gel with a low-pressure Hg lamp. The time of irradiation of the polyoxazoline-based systems was quite long, however. Moreover, only the photoreversible behavior of the polyoxazoline-based systems after a single cycle of irradiation was investigated.
It is very desirable to develop efficient photoreversible hydrogel systems. Indeed, it is very desirable to develop efficient photoreversible polymeric crosslinked systems generally (including hydrophobic systems).